Identification of Small Molecules that inhibit QSOX1 Project Summary The Quiescin Sulfhydryl Oxidase 1 (QSOX1) enzyme is over-produced by multiple types of tumors including pancreas, breast, kidney, lung and prostate. QSOX1 shuttles disulfide bonds in proteins while they are folding in the ER and Golgi. It is especially active in the extracellular matrix (ECM) where it plays a role in tumor cell invasion by helping to fold proteins involved in adherence (laminin) and matrix metalloproteinases. We first targeted QSOX1 by genetically silencing its expression (shRNA) which suppressed tumor growth, invasion and metastasis in vitro and in vivo. This led to the hypothesis that small molecules might also inhibit the enzyme resulting in similar anti-tumor activities. High throughput screening identified several chemical compounds that inhibit the enzymatic activity of QSOX1. The first reported compound was ebselen, a selenium-containing molecule that was shown to covalently bind to Cys-165 and Cys-237 on QSOX1. Treatment of mice bearing human tumor xenografts with ebselen demonstrated suppression of tumor cell growth and invasion compared to controls. Although ebselen was active and inhibited QSOX1, it is somewhat non-specific and also inhibits other enzymes. The second compound, SBI-183, i) inhibits the enzymatic activity of QSOX1 in a cell free assay, ii) binds to QSOX1 with a Kd of 20uM and iii) suppresses tumor cell invasion and metastasis in vitro and in a human tumor xenograft model. Maximal tolerated dose (MTD) studies in mice were >200mg/kg and revealed no pathology in vital organs such as heart, lungs, kidney, and liver. The anti-tumor activity of SBI-183 suggests that identifying more potent analogs of SBI-183 may be useful in the development of anti-tumor/metastatic compounds against QSOX1 as a novel target. This phase I SBIR proposal will employ medicinal chemistry techniques to screen analogs of SBI-183 using a screening funnel that includes i) QSOX1 enzyme inhibition assays, ii) binding assays and iii) in vitro tumor cell invasion assays. Three to five analogs will be identified that will move forward for testing in human xenograft models in a phase II project that includes adsorption, distribution, metabolism, excretion and toxicity (ADMET) studies.